Galileo Has Close Encounter with Pockmarked Jupiter Moon

JANE E. ALLENNovember 5, 1996

LOS ANGELES (AP) _ The first close-up views of Jupiter’s frozen moon Callisto, pockmarked by untold millennia of meteoric assaults, will help determine how it could be so different from its lunar siblings.

Callisto, among four Jupiter moons Italian astronomer Galileo Galilei discovered in 1610, is the oldest, outermost and least geologically active. It is also believed to be one of the most heavily cratered objects in the solar system.

So far, NASA’s Galileo spacecraft has shown that Io has active, sulfur-spewing volcanoes, Europa may have a deep, frozen ocean, and Ganymede has icy quakes.

Galileo came within 686 miles of Callisto’s surface early Monday, taking measurements that should help determine its composition and history.

But scientists will have to wait until late next week, when they begin playing back Galileo’s tape-recorded data of the encounter, to get a look at what the spacecraft found.

With a diameter of 2,986 miles, Callisto is nearly as big as planet-sized Ganymede, the largest moon in the solar system. It also is believed to have a rocky core and a deep, icy crust like Ganymede.

Galileo looked closely at two ancient basins, named Asgard and Valhalla, formed when meteors slammed into Callisto.

``We want to try to understand how the surface has changed since the time of those impacts; what kinds of geologic forces caused those changes to happen,″ said Ken Klaasen, a member of the Galileo imaging science team.

During this near-pass, which lasts a week, Galileo also will get its closest peek yet at the frozen ocean of Europa. But the best close-up of that moon will come Dec. 19 when the spacecraft zooms within 435 miles of the surface.

Galileo, launched from a shuttle in 1989, began an orbital tour of Jupiter and its major moons last December.